1. Field of the Invention
The present invention relates to a displacement detecting device for detecting displacement of a surface-to-be-measured by a non-contact sensor which uses light emitted from a light source.
2. Description of the Related Art
Conventionally, a displacement detecting device is widely used as a device for measuring displacement and shape of a surface-to-be-measured. In the conventional displacement detecting device, the light emitted from a light source is condensed toward the surface-to-be-measured by an objective lens, and the reflected light reflected by the surface-to-be-measured is condensed by an astigmatic optical element and incident on a light receiving element so as to generate a focus error signal by an astigmatic method. Further, by using the focus error signal, a servomechanism is driven to displace the objective lens so that focus position of the objective lens is located on the surface-to-be-measured. At this time, the scale of a linear scale integrally attached to the objective lens through a connection member is read to thereby detect the displacement of the surface-to-be-measured (see, for example, Japanese Unexamined Patent Application Publication No. H05-89480).
In the displacement detecting device disclosed in Japanese Unexamined Patent Application Publication No. H05-89480, in order to improve the accuracy of the displacement detection, the NA (Numerical Aperture) of the objective lens is set large so as to reduce the beam diameter condensed on the surface-to-be-measured. For example, when the beam diameter formed on the surface-to-be-measured is about 2 μm, the detection accuracy of the linear scale will be in a range from several nm to a hundred and several nm.